1. Field of the Invention
The present invention relates to a process process for operating a continuous heterogeneously catalyzed gas phase partial oxidation of at least one organic compound in an oxidation reactor whose charging gas mixture, in addition to the at least one compound to be partially oxidized and molecular oxygen as the oxidant, comprises at least one diluent gas which is substantially inert under the conditions of the heterogeneously catalyzed gas phase partial oxidation, by using air, both as the oxygen source and the inert gas source for the charging gas composition, which has been compressed beforehand in a compressor from a low starting pressure to a higher final pressure.
2. Description of the Background
In this context, a complete oxidation of an organic compound with molecular oxygen means that the organic compound is converted under the reactive action of molecular oxygen in such a way that all of the carbon present in the organic compound is converted to oxides of carbon and all the hydrogen present in the organic compound is converted to oxides of hydrogen. All differing conversions of an organic compound with reactive action of molecular oxygen are grouped together here as partial oxidation of an organic compound. In other words, the term partial oxidation in this document is intended to refer especially also to partial ammoxidations, which are characterized by the organic compound being partially reactively converted in the presence of ammonia.
In particular, partial oxidations in this context are intended to refer to those conversions of organic compounds with the reactive action of molecular oxygen in which the organic compound to be partially oxidized, on completion of conversion, contains at least one more oxygen atom in chemically bonded form than before the partial oxidation was carried out.
In this context, diluent gases which behave substantially inertly under the conditions of the heterogeneously catalyzed gas phase partial oxidation are those diluent gases of whose constituents, under the conditions of the heterogeneously catalyzed gas phase partial oxidation, taking each constituent on its own, more than 95 mol %, preferably more than 99 mol %, remain unchanged.
It is common knowledge that heterogeneously catalyzed partial oxidation of various organic precursor compounds with molecular oxygen in the gas phase allows numerous basic chemicals to be obtained.
Examples include the conversion of xylene to phthalic anhydride, the conversion of propylene to acrolein and/or acrylic acid (cf., for example, DE-A 2351151), the conversion of tert-butanol, isobutene, isobutane, isobutyraldehyde or the methyl ether of tert-butanol to methacrylonitrile or to methacrolein and/or methacrylic acid (cf., for example, DE-A 2526238, EP-A 92097, EP-A 58927, DE-A 4132263, DE-A 4132684 and DE-A 4022212), the conversion of acrolein to acrylic acid, the conversion of methacrolein to methacrylic acid (cf., for example, DE-A 2526238), the conversion of butadiene to maleic anhydride (cf., for example, DE-A2106796 and DE-A 1624921), the conversion of n-butane to maleic anhydride (cf., for example, GB-A 1464198 and GB-A 1291354), the conversion of ethylene to ethylene oxide or of propylene to propylene oxide (cf., for example, DE-B 1254137, DE-A 2159346, EP-A 372972, WO 89/0710, DE-A 4311608 and Beyer, Lehrbuch der organischen Chemie [Textbook of Organic Chemistry], 17 th edition (1973), Hirzel Verlag Stuttgart, p. 261), the conversion of propylene and/or acrolein to acrylonitrile (cf., for example, DE-A 2351151), the conversion of isobutene and/or methacrolein to methacrylonitrile, the oxidative dehydrogenation of hydrocarbons (cf., for example, DE-A 2351151), the conversion of propane to acrylonitrile or to acrolein and/or acrylic acid (cf., for example DE-A 10131297, EP-A 1090684, EP-A 608838, DE-A 10046672, EP-A 529853, WO 01/96270 and DE-A 10028582) etc. The process according to the invention is applicable especially to all of the aforementioned partial oxidations.
The catalysts to be used for such reactions are normally in the solid state.
The catalysts to be used are particularly frequently solid oxide compositions or noble metals (e.g. Ag). In addition to oxygen, the catalytically active oxide composition may contain only one other element or more than one other element (multielement oxide compositions). The catalytically active oxide compositions used are particularly frequently those which include more than one metal, in particular transition metal, element. These are referred to as multimetal oxide compositions.
As a consequence of the generally marked exothermic character of most heterogeneously catalyzed gas phase partial oxidations of organic compounds with molecular oxygen, reaction partners are typically diluted with a gas which is substantially inert under the conditions of the heterogeneously catalyzed gas phase partial oxidation and is capable of absorbing the heat of reaction released with its heat capacity, thus having a favorable effect on the reaction rate.
One of the most frequently used inert diluent gases is molecular nitrogen, which is always used automatically when air is used as a partial oxygen source or exclusively for the heterogeneously catalyzed gas phase partial oxidation.
As a consequence of its general availability, another inert diluent gas which is frequently used is steam. In many cases, cycle gas (which generally also contains oxygen which is yet to be consumed) is also used as an inert diluent gas (cf., for example, EP-A 1180508). Cycle gas refers to the residual gas which remains after a one-stage or multistage (in the multistage heterogeneously catalyzed gas phase partial oxidation of organic compounds, the gas phase partial oxidation, in contrast to the one-stage heterogeneously catalyzed gas phase partial oxidation, is carried out not in one, but rather in at least two, reactors connected in series, in which case oxidants (for example in the form of air) can be supplemented between successive reactors; multiple stages are employed especially when the partial oxidation proceeds in successive steps; in these cases, it is frequently appropriate to optimize both the catalyst and the other reaction conditions to the particular reaction step and to carry out the reaction step in a dedicated reactor, in a separate reaction stage; however, it can also be employed if, for reasons of heat removal or for other reasons (cf., for example, DE-A 19902562), the conversion is spread over a plurality of reactors connected in series; an example of a heterogeneously catalyzed gas phase partial oxidation which is frequently carried out in two stages is the partial oxidation of propylene to acrylic acid; in the first reaction stage, the propylene is oxidized to acrolein and, in the second reaction stage, the acrolein to acrylic acid; correspondingly, the preparation of methacrylic acid (via methacrolein as an intermediate) is usually carried out in two stages starting from isobutene; however, when suitable catalyst charges are used, both aforementioned partial oxidations can also be carried out in one stage (both steps in one reactor); d., for example, EP-A 990636 and EP-A 1106598) heterogeneously catalyzed gas phase partial oxidation of at least one organic compound when the target product is removed more or less selectively (for example by absorption into a suitable solvent; cf., for example, DE-A 19606877) from the product gas mixture. In general, it consists predomionantly of the inert diluent gases used for the partial oxidation, and also of steam typically by-produced in the partial oxidation and carbon oxides formed by undesired complete secondary oxidation. In some cases, it also contains small amounts of oxygen which has not been consumed in the partial oxidation (residual oxygen) and/or unconverted organic starting compounds. Typically, only a portion of the residual gas is used as cycle gas. The remaining amount of residual gas is normally incinerated.
A heterogeneously catalyzed gas phase partial oxidation is normally carried out over a fixed catalyst bed or in a fluidized catalyst bed.
To this end, the starting reaction gas mixture, which consists predominantly of the at least one organic compound to be partially oxidized (typically referred to as precursor compound), molecular oxygen (optionally ammonia in the case of an ammoxidation) and inert diluent gas (including any cycle gas), is generally conducted through the catalyst charge at elevated temperature (generally a few hundred ° C., typically from 100 to 600° C.). The chemical conversion is effected during the contact time on the catalyst surface.
As already mentioned for the cycle gas formation, as a consequence of numerous parallel and subsequent reactions proceeding in the course of the catalytic gas phase partial oxidation, and also as a consequence of the generally used inert diluent gases (under particular conditions, the at least one organic precursor compound can also function as diluent gas, when it is present in excess in the starting reaction gas mixture relative to the molecular oxygen present therein), a heterogeneously catalyzed gas phase partial oxidation does not result in a pure organic target compound being obtained, but rather a reaction gas mixture from which the target product has to be removed.
When the region of the gas phase oxidation forms the actual reaction zone, the product gas mixture, for the purpose of target product removal, is normally fed to what is known as a workup zone in which this removal is effected.
Typically (for example in the case of acrylic acid and in the case of methacrylic acid), the target product removal from the product gas mixture is carried out via extractive, fractionally condensing and/or rectificative separating processes in separating columns containing separating internals, through which the product gas mixture is conducted (cf., for example, DE-A 19606877, DE-A 19631645, EP-A 982289, DE-A 19740253, EP-A 982287, EP-A 1 041 062, EP-A 778 255, EP-A 695 736, DE-A 19 501 325 and EP-A 925 272). The residual gas which remains, as already described, is used, if required, as cycle gas for diluting the starting reaction gas mixture.
To convey the reaction gas mixture through the catalyst charge of the heterogeneously catalyzed partial gas phase oxidation, and also through the downstream workup, a pressure differential is required between reactor inlet and residual gas outlet.
In practice, this pressure differential is typically generated by adjusting the starting reaction gas mixture, before its entry into the oxidation reactor to an elevated pressure compared to the air pressure of the environment. These pressures are typically from 0.2 to 5 bar gauge (bar gauge means elevated pressure compared to normal atmospheric pressure) or more, frequently from 0.5 to 4.5 bar gauge and in many cases from 1 or 2 to 4 bar gauge. High pressures are required in particular when the gas volume to be conveyed is high (for example in the case of high load methods, as described in the document DE-A 19927624, DE-A 19948248, DE-A 19948241, DE-A 19910508, DE-A 10313210, DE-A 10313214, DE-A 10313213 and DE A 19910506), since the latter, for a given reactor and given workup apparatus, also causes an increased pressure drop in the conveyance between the catalyst charge, any intermediate cooler and/or aftercooler charged with random packings, and the workup apparatus.
Since the organic precursor compound to be partially oxidized is in practice frequently stored in liquid form, it is generally sufficient to simply evaporate, in order to bring the organic precursor compound to be partially oxidized to the required reactor inlet pressure. The steam to be used as an inert diluent gas is usually likewise available from highly differing sources with sufficient superatmospheric pressure.
However, this is generally not true for air used as oxygen source (it is typically taken from the atmosphere surrounding the oxidation reactor at atmospheric pressure), the cycle gas (it normally has the reactor inlet pressure minus the pressure drop on the path through the oxidation and through the workup zone) and any other inert diluent gases.
In practice, it is therefore normally necessary to bring at least air used as an oxygen source from a lower starting pressure to a higher final pressure (usually the reactor inlet pressure) by means of a compressor (cf., for example, FIG. 1 of EP-A 990636).
These constituents (for example the oxygen source, air, and the diluent gas source, cycle gas) can be compressed in spatially separated compressors or in a single commpressor (cf. FIG. 1 of EP-A 990636). If required, a plurality of heterogeneously catalyzed gas phase partial oxidation processes can be supplied via an air compressor with compressed air (for example via appropriate feed lines).
The portions of the charging gas mixture (starting reaction gas mixture) which stem from various sources and are substantially at (or brought to) reactor inlet pressure are then, coming from several lines, initially mixed in a, for example static, mixer (generally chambers with internals which generate turbulences), and subsequently optionally heated to inlet temperature and then fed to the oxidation reactor (the entry of the individual gases into the line fed to the static mixer is appropriately selected in such a way (both in sequence and amount) that the formation of explosive mixtures is prevented (in the case of a partial oxidation of propylene to, for example, acrolein and/or acrylic acid, this entry sequence could appropriately be, for example, first cycle gas and/or steam, then crude propene and then air).
In principle, gases could be compressed using compressors of highly differing types. Examples include displacement compressors (for example reciprocating piston compressors, screw compressors and rotary piston compressors), flow compressors (for example turbocompressors, centrifugal compressors, axial compressors and radial compressors) and jet compressors.
Particularly suitable radial compressors are, for example, the Gutehoffnungshütte (GHH) GV10/3L compressor, the Borsig GS900 and GKS450 compressors, the Mannesmann Demag VK80–2 compressor or the Nuovo Pignone SRL1001/B compressor.
The mode of operation of a radial compressor can be illustrated as follows (cf. also DE-A 10259023):
It consists in principle of a housing and at least one rotor which rotates therein, is driven by a drive shapt and is provided with blades. The gas to be compressed enters axially through a suction nozzle. It is deflected radially outward with the centrifugal force of the rotating rotor (closed disk with blades) and thus accelerated to high velocity by the rotor. The function of the housing is to trap the gas and collect it so that it can be further transported through the pressure outlets. The housing simultaneously has the function of converting kinetic energy into pressure. The fact that an increase in cross section reduces the velocity of the gas and thus effects a static pressure increase is generally utilized for this purpose. Various designs of the housing are possible for increasing the cross section. In one-stage compressors or downstream of the last stage of multistage compressors, spiral housings are frequently used. The housing of this type encloses the rotor in spiral form. The cross section increases toward the pressure outlet. The gas flowing through is thus slowed down which implies a simultaneous pressure increase.
Instead of the spiral, it is also possible to use stators, particularly in the case of multistage (e.g. 1- to 3-stage) compressors. The stator is installed in the housing and is in the form of an annular space. It encloses the rotor. Guide blades which form channels with one another which widen continuously in an outward direction are arranged in the stator. In this embodiment, the gas is not accelerated directly into the housing but first flows through the blade channels of the stator. Owing to the widening in the flow direction, they once again produce a decrease in the flow velocity and the consequent pressure build-up.
In multistage compressors, compressed gas (for example air) may advantageously be withdrawn downstream of each compression stage. This then enables, for example, particularly economical compression when the gas to be compressed is required at different pressure levels. The latter is the case, for example, in multistage partial oxidations over the fixed catalyst bed with intermediate air feeding (downstream of the performed oxidation stage) (for example in the two-stage partial oxidation of propylene to acrylic acid). However, it may also be appropriate when a portion of the compressed air is withdrawn in parallel for stripping applications (requires low pressure level).
EP-A 990636 leaves completely open the question of the type of compressor to be used. However, it teaches that the air to be used as an oxygen source merely requires thermal treatment before its compression.
DE-A 10259023 itself considers a thermal treatment of air to be used as an oxygen source to be carried out before the compression not to be necessary. Rather, it is recommended to use a radial compressor as the compressor, since it is said to be largely insensitive toward solid or liquid constituents in the form of very fine particles in the gas to be compressed. This is in particular in view of the fact that when chemical compounds having at least one ethylenically unsaturated double bond (“monomers”) are involved in the heterogeneously catalyzed gas phase partial oxidation, the cycle gas generally also contains monomers (in this context, an ethylenically unsaturated double bond refers to a chemical double bond which is between two carbon atoms and, within the molecule, may be either discrete, isolated from other multiple bonds or conjugated or fused to other multiple bonds; a chemical compound having such a double bond is involved in most heterogeneously catalyzed gas phase partial oxidations (for example in virtually all of those cited at the outset); it may, for example, be the organic precursor compound to be partially oxidized (for example butadiene, propylene, isobutene, acrolein, methacrolein), or the target product (for example acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile) or an intermediate (for example acrolein or methacrolein)). Especially in the case of cocompression of air and cycle gas (according to DE-A 10259023, the compression of the cycle gas and air can be carried out in two separate radial compressors which are driven by two separate motors, or in two compressors which are operated with one motor or in a single compressor which is driven by one motor), polymer particle formation starting from such residual monomers in the course of compression is virtually unavoidable.
However, detailed analyses of long-term experiments have shown that neither the recommendation of DE-A 10259023 nor that of EP-A 990636 is completely satisfactory. Rather, it has been found that, surprisingly, very fine solid and/or liquid particles (having a longest dimension of generally ≦100 μm, frequently from ≧0.1 or ≧0.2 to 50 μm) present in the tiniest amounts in air used as an oxygen source actually have a nonnegligible, adverse effect both in the course of air compression (even when this is carried out together with cycle gas containing residual monomers in one compressor) and in the occurrence of the pressure drop which increases over the operating time when the gas phase partial oxidation is carried out over a fixed catalyst bed. They additionally have an adverse effect on the catalyst performance (activity and/or selectivity).